The halogenation of aromatic compounds is generally known in the art. Halogenated aromatic compounds have found use as flame retardants in various polymer substrates, and processes for their production have become relatively more important.
The difficulty in halogenating aromatics is that the addition of each successive halogen atom deactivates the aromatic nucleus and makes subsequent halogen addition more difficult; for example, the addition of the second chlorine atom to chlorobenzene is more difficult by factor of 8.5:1 when compared with the energy requirement for addition of the first chlorine atom.
In the production of many polybrominated aromatic compounds in which the aromatic nucleus is highly brominated, the reaction is difficult because the product is a solid and tends to agglomerate during the reaction, thus increasing the difficulty of further halogenation that much more.
For example, in a process for producing polybrominated aromatic compounds, several recent patents have suggested the use of fuming sulfuric acid (oleum) as a suitable reaction medium (see U.S. Pat. Nos. 3,232,959 and 3,285,995). The sulfuric acid acts as a solvent; and sulfur trioxide oxidizes the hydrogen bromide back to bromine, thus permitting complete utilization of bromine. However, this process suffers from the difficulty of the product forming solid material which then is very difficult to brominate further. Also, the occlusion of sulfuric acid in the solid product results in impurities which are difficult to remove.
The use of Friedel-Crafts catalysts in the classical approach to brominating aromatic compounds results in the production of dibromo derivatives of the aromatic compounds. Certain improvements have been made; for example, see U.S. Pat. No. 3,285,965 in which the aromatics are brominated in a C.sub.2-4 alkylene dibromide solvent and in the presence of chlorine and a halogenation catalyst. However, up to 20 percent by weight of excess bromine must be used in this process; and the excess bromine is taken up by addition of a C.sub.2-4 alkylene to produce additional dihalide. The process is difficult to control and requires the use of chlorine to oxidize the HBr back to elemental bromine with the subsequent problem of disposing of HCl. Also, the alkylene must be continuously added to make up solvent losses and quench the excess bromine.
In another process, French Patent 69/002,243, the bromination of biphenyl using AlCl.sub.3 catalyst without a solvent is carried out. However, this process suffers from the serious disadvantage of the partially brominated product becoming so viscous that special mixers are required to continue the bromination of the solid material.
In application Ser. No. 120,345 filed Mar. 2, 1971, by Lawrence C. Mitchell now U.S. Pat. No. 3,763,248, an improved method for polybrominated aromatics is described. This application provides a further improvement. More specifically, products produced by the process of this application have less color or lessened tendency to color upon standing than those produced by the process of application Ser. No. 120,345 supra (U.S. Pat. No. 3,763,248). Thus, although the Mitchell process is highly efficacious for bromination of aromatics, the process of the instant invention may be of choice where product specifications require more stringent removal of color forming bodies. A main improvement provided by this invention is the isolation procedure; i.e. isolation of the product via precipitation from the reaction mixture by adding a precipitant, e.g. methanol and the like. Another main improvement are the products produced by the process of this invention.